Photoelectric conversion apparatus and imaging system using the same

ABSTRACT

A photoelectric conversion apparatus includes: a first semiconductor region forming a part of a photoelectric conversion element; a second semiconductor region stacked on the first semiconductor region, and forming a part of the photoelectric conversion element; a third semiconductor region to which a signal charge transferred from the photoelectric conversion element; a fourth semiconductor region of the first conductivity type having an higher impurity concentration, between the first and third semiconductor region and between the second and third semiconductor regions, closer to a main surface than the first semiconductor region, and connected to the first semiconductor region; a first gate electrode over the fourth semiconductor region, an insulating film on the main surface and between the first gate electrode and the fourth semiconductor region; and a second gate electrode between the third and fourth semiconductor regions, and over the insulating film.

TECHNICAL FIELD

The present invention relates to a photoelectric conversion apparatus,in particular to a transfer structure of a signal charge from aphotoelectric conversion element.

BACKGROUND ART

Conventionally, known photoelectric conversion apparatuses include anapparatus which transfers a charge of a photoelectric conversion elementto a floating diffusion region through a transfer MOS transistor andconverts it into a voltage to read out.

Concerning such photoelectric conversion apparatus, Japanese PatentApplication Laid-Open No. 2004-063498 discloses a configuration in whicha signal charge is read out at a low voltage and no signal charges areleft behind. Specifically, it is a photoelectric conversion apparatusthat includes a first gate electrode approximately adjacent to one endof a photodiode region, a second gate electrode adjacent to the firstgate electrode and a drain region approximately adjacent to one end ofthe second gate electrode.

However, according to the configuration disclosed in Japanese PatentApplication Laid-Open No. 2004-063498, since a photodiode is formeddeeply under the first gate electrode, and the photodiode and the drainregion are situated close to each other when the size of pixel isreduced, a punch-through may be possibly produced in a bulk, resultingin an electrically conducting state.

Therefore, an object of the present invention is to provide aphotoelectric conversion apparatus in which electrical connectionbetween a photoelectric conversion element and a drain region can becontrolled easily, and transfer efficiency of a charge from thephotoelectric conversion element can be improved.

DISCLOSURE OF THE INVENTION

A photoelectric conversion apparatus according to the present inventionincludes: a semiconductor substrate having a main surface; a firstsemiconductor region of a first conductivity type forming a part of aphotoelectric conversion element; a second semiconductor region of asecond conductivity type opposite to the first conductivity type beingstacked on the first semiconductor region, and forming a part of thephotoelectric conversion element; a third semiconductor region of thefirst conductivity type to which a signal charge transferred from thephotoelectric conversion element; a fourth semiconductor region of thefirst conductivity type having an impurity concentration higher thanthat of the first semiconductor region, being arranged between the firstand third semiconductor region and being arranged between the second andthird semiconductor regions, closer to the main surface than the firstsemiconductor region, and being connected to the first semiconductorregion; a first gate electrode arranged over the fourth semiconductorregion, an insulating film arranged on the main surface and arrangedbetween the first gate electrode and the fourth semiconductor region;and a second gate electrode arranged between the third and fourthsemiconductor regions, and arranged over the insulating film.

The photoelectric conversion apparatus of the present invention canfacilitate controlling electrical connection between the photoelectricconversion element and the drain region and improve transfer efficiencyof a signal charge.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a planar schematic view illustrating a first embodiment.

FIG. 2 is a cross-sectional schematic view illustrating the firstembodiment.

FIG. 3 is an operational timing chart illustrating the first embodiment.

FIGS. 4A, 4B, 4C and 4D are potential maps illustrating the firstembodiment.

FIG. 5 is an operational timing chart illustrating a second embodiment.

FIGS. 6A, 6B, 6C and 6D are potential maps illustrating the secondembodiment.

FIG. 7 is an operational timing chart illustrating a third embodiment.

FIG. 8 is a planar schematic view illustrating a fourth embodiment.

FIG. 9 is a cross-sectional schematic view illustrating the fourthembodiment.

FIG. 10 is an operational timing chart illustrating the fourthembodiment.

FIGS. 11A, 11B, 11C, 11D, 11E and 11F are potential maps illustratingthe fourth embodiment.

FIG. 12 is a block diagram illustrating an imaging system.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

A photoelectric conversion apparatus according to the present inventionincludes: a photoelectric conversion element having a firstsemiconductor region of a first conductivity type and a secondsemiconductor region of a second conductivity type; and a fourthsemiconductor region of the second conductivity type, connected to thesecond semiconductor region. This fourth semiconductor region has animpurity concentration higher than that of the second semiconductorregion and is arranged closer to the main surface rather than the secondsemiconductor region. Then, the photoelectric conversion apparatusincludes a first gate electrode covering the fourth semiconductor regionand a second gate electrode for controlling electrical connectionbetween the fourth semiconductor region and a third semiconductorregion. Such configuration can improve transfer efficiency of a signalcharge while facilitating separation of the photoelectric conversionelement from a drain region.

Also, at least for a time period during which a signal charge isaccumulated in the photoelectric conversion element, a charge having apolarity opposite to that of the signal charge is accumulated under thefirst gate electrode. Such configuration can reduce mixing of darkcurrent due to a defect of an interface of a semiconductor substrate.

Note that an outer edge of a semiconductor region can be determined in amanner as follows. For example, if a region around the semiconductorregion has a conductivity type opposite to that of the semiconductorregion itself, the outer edge is defined as points where respective netimpurity concentrations become near zero. This outer edge can beconfirmed by measuring with an SCM (scanning capacitance microscope).The net impurity concentration means a difference in concentrationbetween an N-type impurity and a P-type impurity. Then, the depth of asemiconductor region can be defined as the depth at which a peak valueof the impurity concentration exists. Also, the downward direction ordepth direction is defined as the direction from a main surface of asemiconductor substrate having a light receiving surface toward thesemiconductor substrate.

Now, embodiments are hereinafter described with reference to thedrawings. A configuration of each embodiment can be appropriatelycombined with each other. In embodiments, a signal charge is anelectron, a first conductivity type is an N-type semiconductor and a MOStransistor is of N-type, but the signal charge may be, of course, anelectron hole and the conductivity type may be an opposite type.

(First Embodiment)

Firstly, a pixel to which the present invention may be applied isdescribed using FIG. 1. FIG. 1 is a planar schematic view illustratingan element as a block. A pixel 100 includes a photoelectric conversionelement 101, a charge storing portion 102, a floating diffusion region103 and a charge transfer portion 104. Other element is collectivelydesignated simply by the reference number 105. The other elementincludes, for example, a MOS transistor for amplification and a MOStransistor for resetting and may have any detailed configuration. Also,an element isolation region is omitted. The pixel is the smallestrepeating unit that has at least one photoelectric conversion element,and the pixel 100 is arranged in one dimension or two dimensions to forman imaging region. FIG. 1 illustrates three arrays of the pixel 100.FIG. 2 illustrates a cross-sectional view of such pixel 100 taken alongthe lines A-B.

FIG. 2 is the cross-sectional schematic view taken along the lines A-Bin FIG. 1. FIG. 2 illustrates a well 201, a first semiconductor region202 of a first conductivity type, a second semiconductor region 203 of asecond conductivity type, a first gate electrode 204 and a fourthsemiconductor region 205 of the first conductivity type. Further, asecond gate electrode 206, a third semiconductor region 207 of the firstconductivity type, an electrical conductor 208 forming a contact and aninsulating film 209 covering the element are illustrated. A gateinsulating film 210 and an element isolation region 211 for STI and thelike are also shown. On a semiconductor substrate 200, the photoelectricconversion element is arranged, and a main surface 212 of thesemiconductor substrate includes a light receiving surface of thephotoelectric conversion element 101. The dotted lines show the positionof a surface including the main surface 212. Then, the downwarddirection or depth direction is defined as the direction from the mainsurface 212 toward the semiconductor substrate 200. The well 201 may beof the first conductivity type or of the second conductivity type, orthe semiconductor substrate 200.

The second semiconductor region 203 is stacked on the firstsemiconductor region 202, and the first and second semiconductor regions202 and 203 have a P-N junction interface therebetween and form a partof the photoelectric conversion element 101. The third semiconductorregion 207 functions as the floating diffusion region, and a signalcharge of the first semiconductor region 202 is transferred through thefourth semiconductor region 205 by the second gate electrode 206. Thethird semiconductor region 207 and the fourth semiconductor region 205are spaced away from each other, in which the well 201 is arranged. Thefourth semiconductor region 205 is arranged at the same depth as thethird semiconductor region 207, or the fourth semiconductor region 205is arranged closer to the main surface 212 rather than the thirdsemiconductor region 207. Then, the fourth semiconductor region 205 hasan impurity concentration higher than that of the first semiconductorregion 202, and the fourth semiconductor region 205 is arranged closerto the main surface 212 than the first semiconductor region 202 andconnected to the first semiconductor region 202. It may be said that thefourth semiconductor region 205 and the first semiconductor region 202are integrally formed, or their semiconductor regions are continuouslyarranged. Their upper portions are covered by the first gate electrode204. Such configuration can allow the thickness in the depth directionof the fourth semiconductor region 205 to be thinner and prevent adepletion layer from coming close in the depth where the gate cannotcontrol. That is, even if the space between the fourth semiconductorregion 205 and the third semiconductor region 207 are made narrower dueto a reduced size of pixel, the configuration can separate the fourthsemiconductor region 205 from the third semiconductor region 207 andfacilitate controlling electrical connection. Also, a signal charge ofthe first semiconductor region 202 can be effectively transferred to thethird semiconductor region 207. Further, the fourth semiconductor regionhaving a higher impurity concentration is arranged closer to the mainsurface 212 rather than the second semiconductor region, and accordinglyat a comparatively low depletion voltage, many charges can be stored inan n+ region, thus expecting improvement in the number of saturationsignals.

Then, drive of the photoelectric conversion apparatus is described usingan operational timing chart illustrated in FIG. 3. A control signaldesignated by the symbol φ204 controls the first gate electrode 204 andis a voltage supplied to the first gate electrode 204. A control signaldesignated by the symbol φ206 controls the second gate electrode 206 andis a voltage supplied to the second gate electrode 206. In thisembodiment, a low level that the control signal φ204 and the controlsignal φ206 may take is set to −1 V, and a high level is set to 5 V. Thevoltage (207) schematically shows change in voltage of the thirdsemiconductor region 207. A state (207) shows operation performed in thethird semiconductor region, and a state (101) shows a state of thephotoelectric conversion element 101, showing here an accumulationperiod. Timing is shown at from t1 to t9. The drive is hereinafterdescribed.

At t1, the photoelectric conversion element 101 accumulates a signalcharge. The third semiconductor region 207 is supplied with a desiredvoltage (reset voltage) and is at 5 V. A resetting operation is definedas supplying the desired voltage to the third semiconductor region 207,and the desired voltage is 5 V. Next, at t2, an electrical potential ofthe third semiconductor region 207 after being reset is read out. Thesignal from the third semiconductor region 207 after being reset can beused as a noise signal including a noise component at resetting. Thenoise signal is superimposed on the signal based on the signal charge.Then, the noise component can be removed by subtracting the noise signalfrom the signal based on the signal charge. The control signal φ204 andthe control signal φ206 have been at the low level from t1. Duringaccumulation, the control signal φ204 is at the low level, andaccordingly a charge (electron hole) of a polarity opposite to that ofthe signal charge is accumulated under the first gate electrode 204,thus being able to reduce dark current from the main surface on thefourth semiconductor region 205, that is, the semiconductor substrate.In this embodiment, the control signal φ204 is always set to the lowlevel, such that dark current can be always reduced. Also, the controlsignal φ206 is at the low level, thus sufficiently separating the fourthsemiconductor region 205 from the third semiconductor region 207 andalso suppressing dark current produced on a surface of the semiconductorsubstrate under the second gate electrode.

Next, during from t3 to t4, the control signal φ206 goes to the highlevel, thereby the fourth semiconductor region 205 is electricallyconnected to the third semiconductor region 207, and the signal chargeproduced in the photoelectric conversion element 101 is transferred tothe third semiconductor region 207. Then the control signal φ206 returnsto the low level, thus stopping transferring the signal charge from thephotoelectric conversion element 101 to the third semiconductor region207. That is, the photoelectric conversion element 101 terminates oneaccumulation period at t4 and enters a next accumulation period.

While the control signal φ206 is at the high level, the voltage of thethird semiconductor region 207 lowers by a voltage ΔVsig of the signalcharge transferred from the photoelectric conversion element 101. Then,during from t4 to t5, a signal based on the voltage ΔVsig is output as avideo signal. Specifically, if the third semiconductor region 207 isconnected to a gate electrode of a MOS transistor for amplification, asignal based on an electrical potential of the third semiconductorregion 207 including the voltage ΔVsig is output as a source electricalpotential of the MOS transistor for amplification. Then, at t5, thethird semiconductor region 207 is reset to return to the state at t1.The drive after t6 is similar to the above.

Next, a potential state of a semiconductor region under the drivementioned above is described using FIGS. 4A, 4B, 4C and 4D. FIGS. A, 4B,4C and 4D schematically illustrate a potential state relative to asignal charge of each semiconductor region at some timing point. Apotential 202 corresponds to that of the first semiconductor region 202,a potential 204 corresponds to that of the fourth semiconductor region205 under the first gate electrode 204, a potential 206 corresponds tothat of the well under the second gate electrode 206, and a potential207 corresponds to that of the third semiconductor region 207. FIG. 4Aillustrates an initial state, in which a signal charge is not producedin the photoelectric conversion element 101. FIG. 4B illustrates a statecorresponding to that during from t1 to t3 in FIG. 3, and FIG. 4Cillustrates a state corresponding to that during from t3 to t4 in FIG.3, and FIG. 4D illustrates a state corresponding to that during from t4to t5 in FIG. 3. The signal charge is shown by the hatched lines. Thepotential of each semiconductor region in each state is illustrated bypotentials from L41 to L48.

In FIG. 4A, the fourth semiconductor region 205 has an impurityconcentration higher than that of the first semiconductor region 202,and accordingly the fourth semiconductor region 205 has the potentialL43 lower than the potential L42 of the first semiconductor region 202.Then, although the potential under the second gate electrode is thepotential L41, it may be higher. In FIG. 4B, a signal charge produced inthe photoelectric conversion element 101 is accumulated. The fourthsemiconductor region 205 has the impurity concentration higher than thatof the first semiconductor region, and therefore the fourthsemiconductor region 205 can store the signal charge more by adifference between the signal charge held at the potential L42 and thesignal charge held at the potential L43 than the case where the fourthsemiconductor region 205 has the same impurity concentration as thefirst semiconductor region. In FIG. 4C, a voltage of the high level issupplied to the second gate electrode 206, thus lowering the potentialof the well under the second gate electrode 206 to the potential L47.Here, between the first semiconductor region 202 and the thirdsemiconductor region 207, a step-like potential is formed, thereby thesignal charge is smoothly transferred to the third semiconductor region207. In FIG. 4D, a voltage of the low level is supplied to the secondgate electrode 206, and accordingly the potential of the well under thesecond gate electrode 206 goes to the potential L41, completing thetransfer of the signal charge. Subsequently, accumulation of a signalcharge in the photoelectric conversion element 101 starts again. DuringFIGS. 4C and 4D, the potential of the fourth semiconductor region 205becomes higher than that of the well under the second gate electrode206. Such relationship between the potentials can reduce a residualsignal charge in the fourth semiconductor region 205.

As described above, the configuration as shown in this embodiment canimprove transfer efficiency while suppressing electrical connectionbetween the second semiconductor region and the third semiconductorregion forming a photoelectric conversion element during an undesiredtime period. Because the fourth semiconductor region can be formedcloser to the main surface, that is, shallowly, the fourth semiconductorregion is easily connected to a passage of the signal charge, createdunder the second gate electrode on transferring the signal charge. Thatis, because of a higher transfer efficiency, the drive at a lowervoltage can be performed, compared to a configuration to which thisconfiguration is not applied.

Also, the electrical potential of the first gate electrode 204 is set ina manner that an electron hole is accumulated in the fourthsemiconductor region under the first gate electrode 204, which canreduce dark current from the main surface.

Also, in this embodiment, the first gate electrode is always set to acertain electrical potential, and accordingly the first gate electrodecan drive the entire pixels in common. Therefore, gate electrodes of theentire pixels can be connected to the same control line, thus reducingthe number of control lines and a control circuit. Further, the firstgate electrode can be also provided continuously across a plurality ofpixels. The structure such that the first gate electrode is providedcontinuously is a structure wherein the gate electrode formed from thesame material extends over a plurality of pixels. Such configuration canreduce the size of pixel. Note that the first gate electrode and thesecond gate electrode are connected to a different control line,respectively.

(Second Embodiment)

This embodiment differs from the first embodiment in the control signalφ204. Also, in the configuration of the first embodiment, the firstsemiconductor region differs from the fourth semiconductor region in therelation between their potentials. FIG. 5 illustrates an operationaltiming chart, and FIG. 6 schematically illustrates a potentialcorresponding to a signal charge in each semiconductor region when driveis carried out according to FIG. 5. FIG. 5 corresponds to FIG. 3, FIGS.6A, 6B, 6C and 6D correspond to FIGS. 4A, 4B, 4C and 4D, and a likefunction is designated by a like symbol and description thereof isomitted.

Firstly, in FIG. 5, the control signal φ204 is not always at a low leveland goes to a high level at t3 (and at t7). The control signal φ204 goesto the high level on transferring a signal charge from the photoelectricconversion element 101, thus being able to suppress formation of apotential to block on transferring the signal charge and improvetransfer efficiency. Other operation of the drive similar to FIG. 3 isnot described.

Next, the transfer operation is described in detail using FIGS. 6A, 6B,6C and 6D. A like state to FIGS. 4A, 4B, 4C and 4D is not described.FIG. 6A illustrates an initial state, in which a signal charge is notproduced in the photoelectric conversion element 101. FIG. 6Billustrates a state corresponding to that during from t1 to t3 in FIG.5, FIG. 6C illustrates a state corresponding to that during from t3 tot11 in FIG. 5, and FIG. 6D illustrates a state corresponding to thatduring from t4 to t5 in FIG. 5. The signal charge is designated by thehatched lines. A potential of each semiconductor region in each state isshown by potentials from L61 to L69.

In FIG. 6A, a potential of the fourth semiconductor region 205 goes tothe potential L62, being higher than the potential L63 of the firstsemiconductor region 202. A voltage of the first gate electrode 204, andthe impurity concentration of the fourth semiconductor region 205 andthe depth of the impurity concentration may possibly cause such relationbetween the potentials. Of course, the relation between the potentialsmay be similar to that in the first embodiment. In FIG. 6B, a signalcharge is accumulated. In the case of the relation between thepotentials in this embodiment, the amount of the signal charge capableof being held becomes less than that in the first embodiment. In FIG.6C, because the control signal φ206 is at the high level, a potential ofthe well under the second gate electrode 206 changes from the potentialL61 to the potential L68. The control signal φ204 also goes to the highlevel, and the potential of the fourth semiconductor region 205 goesfrom the potential L62 to the potential L66 lower than the potentialL63. Such operation creates a step-like potential between the firstsemiconductor region 202 and the third semiconductor region 207, thusbeing able to effectively transfer the signal charge. In FIG. 6D, thecontrol signal φ206 goes to the low level, and the transfer of thesignal charge ends.

Now, operation in FIGS. 6C and 6D, that is, operation at t11 between t3and t4 in FIG. 5 is described. At t3 in FIG. 5, both the control signalφ204 and the control signal φ206 are at the high level. Then, at t11 inFIG. 5, the control signal φ204 goes to the low level, and the controlsignal φ206, subsequently, goes to the low level. That is, the potentialof the fourth semiconductor region 205 is forced to return from thepotential L66 to the potential L62, and subsequently the potential ofthe well under the second gate electrode 206 is forced to go from thepotential L68 to the potential L61. Such operation can transfer thesignal charge to the third semiconductor region 207 with no signalcharges being left behind in the fourth semiconductor region 205. Notethat if the control signal φ206 is made to go to the low level beforethe control signal φ204, the potential L61 forming a barrier is createdbetween the potential L66 of the fourth semiconductor region 205 and thethird semiconductor region 207, and accordingly the signal charge islikely to be left behind in the fourth semiconductor region.

As mentioned above, the control signal φ204 is forced to go to the highlevel on transferring the signal charge, thus being able to more improvetransfer efficiency, compared to the first embodiment. Of course, thedriving method of this embodiment can be also applied to theconfiguration having the relation between the potential of the firstsemiconductor region 202 and the potential of the fourth semiconductorregion 205 as shown in FIG. 4A.

(Third Embodiment)

This embodiment differs from the second embodiment in that the low levelof the control signal φ204 differs from that of the control signal φ206.In this embodiment, the low level of the control signal φ204 was set to−3 V and the low level of the control signal φ206 was set to −1 V. Thatis, the low level of the voltage supplied to the second gate electrodeis higher than that of the voltage supplied to the first gate electrode.Such relation between the voltages can improve a breakdown voltagebetween the second gate electrode 206 and the third semiconductor region207. It is because of the reason as follows. The third semiconductorregion 207 is set to a high electrical potential on resetting. At thistime, if a voltage of the low level is supplied to the first gateelectrode 204 and the second gate electrode 206, an electric fieldbetween the second gate electrode 206 and the third semiconductor region207 adjacent to each other may become large. Then, the voltage of thelow level of the second gate electrode 206 is made higher than that ofthe first gate electrode 204, which can reduce a voltage between thesecond gate electrode 206 and the third semiconductor region 207adjacent to each other. Accordingly, mixing of dark current can bereduced while maintaining a dielectric voltage.

Next, this embodiment is described in detail using an operational timingchart in FIG. 7. During an accumulation period of the photoelectricconversion element, the control signal φ204 and the control signal φ206are at the low level. A value of the control signal φ204 at this timehas a voltage value lower than that of the control signal φ207 (higheron the negative side). In other words, a value of the control signal ofthe low level supplied to the second gate electrode takes a valuebetween a value of the control signal of the low level supplied to thefirst gate electrode and a value of the control signal of the high levelsupplied to the second gate electrode. Supplying the voltages in such amanner can reduce mixing of the dark current and maintain the breakdownvoltage of the second gate electrode. Of course, the driving method ofthis embodiment can be applied to the configuration having the relationbetween the potential of the first semiconductor region 202 and thepotential of the fourth semiconductor region 205 as shown in FIG. 4A.

Fourth Embodiment

This embodiment differs from the first embodiment in a configuration ofpixel. FIG. 8 is a planar schematic view illustrating an element as ablock. FIG. 8 illustrates photoelectric conversion elements 801, 804,813 and 816, charge storing portions 802, 805, 812 and 815, chargetransfer portions 803, 806, 812 and 815, and a floating diffusion region807. Other element is collectively illustrated by the reference number808. The other element includes, for example, a MOS transistor foramplification and a MOS transistor for resetting, a configuration ofwhich may be of any type. A pixel unit 800 includes the fourphotoelectric conversion elements 801, 804, 813 and 816, which share aread out circuit 808. That is, it can be said that the pixel unit 800includes four pixels. Each pixel has the photoelectric conversionelement, the charge storing portion and the charge transfer portion. Forexample, a first pixel has the photoelectric conversion element 801, thecharge storing portion 802 and the charge transfer portion 803. A secondpixel has the photoelectric conversion element 804, the charge storingportion 805 and the charge transfer portion 806. A third pixel has thephotoelectric conversion element 813, the charge storing portion 812 andthe charge transfer portion 811. A fourth pixel has the photoelectricconversion element 816, the charge storing portion 815 and the chargetransfer portion 814. The pixel unit 800 can be divided into a group ofthe photoelectric conversion elements 801 and 804, and a group of thephotoelectric conversion elements 813 and 816. Then, the charge transferportions 806, 811 and the charge storing portions 805, 812 are shared bythe two photoelectric conversion elements, respectively. FIG. 9illustrates a cross-sectional view of such pixel unit 800, that is, onegroup thereof, taken along the lines A-B.

FIG. 9 is the cross-sectional schematic view taken along the lines A-Bof FIG. 8. FIG. 9 illustrates a well 901, a first semiconductor region902 of a first conductivity type, a second semiconductor region 903 of asecond conductivity type, a first gate electrode 904, a fourthsemiconductor region 905 of the first conductivity type, a second gateelectrode 906, and a third semiconductor region 910. Further, a thirdgate electrode 907, a fifth semiconductor region 908 of the firstconductivity type and a fourth gate electrode 909 are illustrated. Anelectrical conductor 911 forms a contact, and an insulating film 912covers the elements. A gate insulating film is designated by thereference number 213, and an element isolation region for STI and thelike is designated by the reference number 214. Although not shown here,a semiconductor region of the first conductivity type of thephotoelectric conversion element 804 is also similar to that of thefirst semiconductor region 902. On a semiconductor substrate 900, thephotoelectric conversion elements are arranged, and a main surface 915of the semiconductor substrate includes a light receiving surface of thephotoelectric conversion element 801. The position of a surfaceincluding the main surface 915 is shown by the dotted lines. Thedownward or depth direction is defined as the direction from the mainsurface 915 toward the semiconductor substrate 900.

The first semiconductor region 902 and the second semiconductor region903 have a P-N junction interface and form a part of the photoelectricconversion element 101. The third semiconductor region 910 functions asthe floating diffusion region. The fifth semiconductor region 908 isarranged between the fourth semiconductor region 905 and the thirdsemiconductor region 910. Then, a signal charge of the firstsemiconductor region 902 is transferred from the fourth semiconductorregion 905 to the fifth semiconductor region 908 by the second gateelectrode 906 and further transferred to the third semiconductor region910 by the fourth gate electrode 909. Then, the fourth semiconductorregion 905 and the fifth semiconductor region 908 have an impurityconcentration higher than that of the first semiconductor region 902,and also, the fourth semiconductor region 905 and the fifthsemiconductor region 908 are positioned closer to the main surface 212rather than the first semiconductor region 902. The positional relationbetween the fourth semiconductor region 905 and the fifth semiconductorregion 908, and the semiconductor region of the first conductivity typeat the photoelectric conversion element 804 is also similar. Then, thefirst semiconductor region 902 and the fourth semiconductor region 905are electrically connected to each other, and the fifth semiconductorregion 908 and the semiconductor region of the first conductivity typeof the photoelectric conversion element 804 are electrically connectedto each other. In other words, they are integrally formed orcontinuously arranged. The first gate electrode 904 covers an upperportion of the fourth semiconductor region 905, and the third gateelectrode 907 covers an upper portion of the fifth semiconductor region.Such configuration can reduce the number of elements per photoelectricconversion element, compared to the configuration of the firstembodiment. Also, the fourth semiconductor region 905 and the fifthsemiconductor region 908 having a higher impurity concentration arepositioned closer to the main surface 915 rather than the secondsemiconductor region 902. Such configuration can effectively transferthe signal charge while facilitating controlling electrical connectionand disconnection between the fifth semiconductor region 908 and thethird semiconductor region 910. Also, at a comparatively low depletionvoltage, many signal charges can be stored in an n+ region, thusexpecting improvement in the number of saturation signals. Further, thetotal area of the same node portion as the floating diffusion region canbe provided at an equivalent level to that of the first embodiment. Forexample, when forming an input portion of a MOS transistor foramplification, voltage sensitivity per signal charge can be maintainedas is conventionally done.

Next, one example of drive of such photoelectric conversion apparatus isdescribed using an operational timing chart in FIG. 10. A control signalφ904 controls the first gate electrode 904 and is a voltage supplied tothe first gate electrode 904. A control signal φ906 controls the secondgate electrode 906 and is a voltage supplied to the second gateelectrode 906. A control signal φ907 controls the third gate electrode907 and is a voltage supplied to the third gate electrode 907. A controlsignal φ909 controls the fourth gate electrode 909 and is a voltagesupplied to the fourth gate electrode 909. In this embodiment, a lowlevel of the control signal φ904 and the control signal φ907 is set to−3 V and a high level is set to 5 V, and a low level of the controlsignal φ906 and the control signal φ909 is set to −1 V and a high levelis set to 5 V. A voltage (910) schematically illustrates change involtage of the third semiconductor region 910. A state (910) illustratesoperation performed in the third semiconductor region, and a state(shutter) illustrates a state of a mechanical shutter in an imagingsystem, representing exposing and light shielding. Timing is illustratedat from t1 to t14. The drive is hereinafter described. Like operation tothe first embodiment is not described.

Firstly, at t1, the photoelectric conversion element 801 and thephotoelectric conversion element 804 have accumulated a signal charge.The third semiconductor region 910 has been at 5 V due to a desiredvoltage (reset voltage) being supplied. Next, at t2, the mechanicalshutter is closed to shield against light. At t3, an electricalpotential of the third semiconductor region 910 after being reset isread out. The read out signal can be used as a noise signal in a signalfrom the photoelectric conversion element 804. Then, during a timeperiod from t1 to t3, the control signal φ904, the control signal φ906,the control signal φ907 and the control signal φ907 are at the lowlevel. Because the control signal φ904 is at the low level, a charge(electron hole) having a polarity opposite to the signal charge isaccumulated under the first gate electrode 904, thus being able toreduce dark current from the main surface on the fourth semiconductorregion 905. Also, because the control signal φ907 is at the low level, acharge (electron hole) having a polarity opposite to the signal chargeis accumulated under the third gate electrode 907, thus being able toreduce dark current from the main surface on the fifth semiconductorregion 908. In this embodiment, the control signal φ904 is always set tothe low level, which can always reduce dark current. Also, the controlsignal φ906 and the control signal φ909 are set to the low level,thereby the fourth semiconductor region 905 is sufficiently separatedfrom the fifth semiconductor region 908, and the fifth semiconductorregion 908 is sufficiently separated from the third semiconductor region910. At t4, the control signal φ907 and the control signal φ909 go tothe high level. Then, the signal charge is transferred from thephotoelectric conversion element 804 to the third semiconductor region910, and a voltage of the third semiconductor region 910 changes by avoltage ΔVsig. At t6, the transfer of the signal charge of thephotoelectric conversion element 804 ends, and a signal including thesignal charge ΔVsig of the photoelectric conversion element 804 is readout from the third semiconductor region 910. At t5 and t6, the controlsignal φ907 and the control signal φ909 go to the low level in thisorder, which can reduce a signal charge that is not transferred.

Next, at t7, the third semiconductor region 910 is reset and the voltageof the third semiconductor region 910 goes to 5 V. Subsequently, readingout the signal charge of the photoelectric conversion element 801 isstarted. At t8, the control signal φ906 goes to the high level, and thesignal charge is transferred from the first semiconductor region 902 andthe fourth semiconductor region 905 to the fifth semiconductor region908. During a time period from t1 to t7, the control signal φ904 is atthe low level so as to reduce dark current, and accordingly the darkcurrent has a little effect on the signal charge. Also, the fourthsemiconductor region 905 is provided, which can improve transferefficiency and lower the voltage supplied to the control signal φ906 asthe high level. At t9, the electrical potential of the thirdsemiconductor region 910 after being reset is read out. The read outsignal can be used as a noise signal in the signal from thephotoelectric conversion element 801. Also, the control signal φ907 goesto the high level and the signal charge is transferred to the fifthsemiconductor region 908 and held. Because there is not a potentialbarrier between the photoelectric conversion element 804 and the chargestoring portion 805, the photoelectric conversion element 804, onstoring the signal charge, can be also used to store the charge inaddition. Also in the case of many signal charges, the signal charge canbe sufficiently held. At t10, the control signal φ909 goes to the highlevel, and the signal charge is transferred from the fifth semiconductorregion 908 to the third semiconductor region 910. Here, because thecontrol signal φ906, the control signal φ907 and the control signal φ909are at the high level, an area between the second semiconductor regionand the third semiconductor region is put in an electrically conductingcondition and a step-like relation between the potentials is formed,then, the signal charge can be effectively transferred. During from t11to t13, the control signal φ906, the control signal φ907 and the controlsignal φ909 go to the low level in turn, which can reduce a signalcharge that is not transferred and improve transfer efficiency to thethird semiconductor region 910. Of course, the control signal φ906, thecontrol signal φ907 and the control signal φ909 may simultaneously go tothe low level. Here, the voltage of the third semiconductor region 910changes by a voltage ΔVsig2 during a time period from t10 to t13, inwhich, at t10, transfer of the signal charge to the third semiconductorregion 910 starts. At t13, a signal based on the electrical potential ofthe third semiconductor region 910 is output, and a signal including thevoltage ΔVsig2 is read out. At t14, the third semiconductor region 910is reset and returns to the state before exposing, and the mechanicalshutter is opened (exposing), assuming the state at t1. Then, even ifthe control signal φ907 and the control signal φ909 simultaneously go tothe high level at t10, the transfer can be carried out. Also, if notoverlapped with the time period for reading out the noise signal, thecontrol signal φ906, the control signal φ907 and the control signal φ909after t8 may simultaneously go to the high level.

Next, a potential state of a semiconductor region under such drive isdescribed using FIGS. 11A, 11B, 11C, 11D, 11E and 11F. FIGS. 11A, 11B,11C, 11D, 11E and 11F schematically illustrate a potential of eachsemiconductor region relative to a signal charge at some timing point. Apotential 902 corresponds to that of the first semiconductor region 902,a potential 904 corresponds to that of the fourth semiconductor region905 under the first gate electrode 904, and a potential 906 correspondsto that of a well under the second gate electrode 906. A potential 907corresponds to that of the fifth semiconductor region 908 under thethird gate electrode 907, a potential 909 corresponds to that of a wellunder the fourth gate electrode 909, and a potential 910 corresponds tothat of the third semiconductor region 910. FIG. 11A illustrates aninitial state, in which a signal charge is not produced in thephotoelectric conversion element 801 and the photoelectric conversionelement 804. FIG. 11B illustrates a state corresponding to that duringfrom t1 to t3 in FIG. 10, FIG. 11C illustrates a state corresponding tothat during t6 in FIG. 10 and FIG. 11D illustrates a state correspondingto that during t9 in FIG. 10. FIG. 11E illustrates a state correspondingto that during t12 in FIG. 10 and FIG. 11F illustrates a statecorresponding to that during t13. The signal charge is shown by thehatched lines. A potential of each semiconductor region in each state isshown by potentials from L101 to L112.

In FIG. 11A, because the fourth semiconductor region 905 and the fifthsemiconductor region 908 have an impurity concentration higher than thatof the first semiconductor region 902, the fourth semiconductor region905 and the fifth semiconductor region 908 have the potential L103 lowerthan the potential L102 of the first semiconductor region 902. Then,although a potential under the second gate electrode 906 and the fourthgate electrode 907 is the potential L101, it may be a higher potential.Then, to each gate electrode, a voltage of the low level is supplied. InFIG. 11B, a signal charge produced in the photoelectric conversionelement 801 is accumulated in the second semiconductor region 902 andthe fourth semiconductor region 905. At the same time, a signal chargeproduced in the photoelectric conversion element 804 is accumulated inthe fifth semiconductor region 908 and the semiconductor region (notshown) of the first conductivity type forming the photoelectricconversion element 804. Then, for simplicity, assuming that thephotoelectric conversion element 801 and the photoelectric conversionelement 804 have the same amount L105 of the signal charge.Subsequently, the signal charge from the photoelectric conversionelement 804 is read out. Between FIGS. 11B and 11C, a voltage of thehigh level is supplied to the fourth gate electrode 909 and a potentialof the well under the fourth gate electrode 909 is lowered, and thesignal charge is transferred from the fifth semiconductor region 908 tothe third semiconductor region 910. FIG. 11C illustrates a state in thatthe voltage of the low level is supplied to the fourth gate electrode909. The signal charge of the photoelectric conversion element 804 heldin the fifth semiconductor region 908 is held in the fourthsemiconductor region. Here, the signal charge held in the fifthsemiconductor region 908 can be also transferred completely to the thirdsemiconductor region 910. Subsequently, reading out the signal charge ofthe photoelectric conversion element 801 is started.

In FIG. 11D, a voltage of the high level is supplied to the second gateelectrode 906 and the third gate electrode 907. Then, the potential ofthe well under the second gate electrode 906 has changed from thepotential L101 to the potential L107, and the potential of the fifthsemiconductor region 908 has changed from the potential L103 to thepotential L109. At this time, a potential between the fourthsemiconductor region 905 and the fifth semiconductor region 908 has beenmade step-like, and the signal charge of the photoelectric conversionelement 801 held in the second semiconductor region 902 and the fourthsemiconductor region 905 is effectively transferred to the fifthsemiconductor region 905. In FIG. 11E, a voltage of the high level issupplied to the fourth gate electrode 909, and the potential of the wellunder the fourth gate electrode 909 lowers from the potential L101 tothe potential L111. Here, a step-like potential is formed between thefifth semiconductor region 908 and the third semiconductor region 910,and thus effectively transferring the signal charge from the fifthsemiconductor region 908 to the third semiconductor region 910. In FIG.11F, a voltage of the low level is supplied to the fourth gate electrode909 and the potential of the well under the fourth gate electrode 909goes to the potential L101, and the transfer of the signal charge ends.

As mentioned above, the configuration of this embodiment can reduce thenumber of elements compared to the configuration of the firstembodiment. Further, the fourth semiconductor region 905 having a higherimpurity concentration is arranged closer to the main surface 915 ratherthan the second semiconductor region 902. This configuration caneffectively transfer the signal charge while facilitating controllingelectrical connection and disconnection between the fourth semiconductorregion 905 and the fifth semiconductor region 908. Also, the fifthsemiconductor region 908 having a higher impurity concentration isarranged closer to the main surface 915 rather than the semiconductorregion (not shown) of the first conductivity type forming thephotoelectric conversion element 804. This configuration can effectivelytransfer the signal charge while facilitating controlling electricalconnection and disconnection between the fifth semiconductor region 908and the third semiconductor region 910. Therefore, the transferefficiency of the signal charge from the first semiconductor region 902to the third semiconductor region 910 can be improved. Also, at acomparatively low depletion voltage, many signal charges can be storedin the n+ region, thus expecting improvement in the number of saturationsignals.

Further, the first gate electrode 904 and the third gate electrode 907are set to an electrical potential so as to accumulate an electron holeunder respective gate electrodes, which can reduce dark current from themain surfaces on the fourth semiconductor region 905 and the fifthsemiconductor region 908. Then, the first gate electrode 904 may bealways set to a certain electrical potential, and accordingly a gateelectrode connected in common to the entire pixels can be used. Commonconnection to the entire pixels can reduce the number of control linesand a control circuit, and further, a gate electrode can be provided incommon, which can reduce the size of pixel. Of course, the first gateelectrode 904 may be driven in the manner of the second embodiment.

Then, in this embodiment, the drive, in the case where the mechanicalshutter is used, has been described. When the signal charge of thephotoelectric conversion element 801 is read out under the drive asshown in FIG. 10, providing the mechanical shutter can reduce an effectof the signal charge produced in the photoelectric conversion element804. But the mechanical shutter may be optionally used.

Also, although, in this embodiment, the configuration has been describedin which the pixel unit includes the four photoelectric conversionelements, a configuration may be such that the pixel unit includes twophotoelectric conversion elements, that is, the two photoelectricconversion elements 801 and 804, and the other circuit 808. The pixelunit may have any number of photoelectric conversion elements.

(Application to Imaging System)

This embodiment is described, using FIG. 12, about the cases where animaging system adopts the photoelectric conversion apparatus describedin from the first embodiment to the fourth embodiment. The imagingsystem includes a digital still camera, a digital video camera, and adigital camera for a mobile phone.

FIG. 12 illustrates a configuration of a digital still camera. Anoptical image of an object is formed on an imaging area of aphotoelectric conversion apparatus 1204 through an optical systemincluding a lens 1202. A barrier 1201 having both a protection functionof the lens 1202 and a main switch function may be provided outside ofthe lens 1202. A diaphragm 1203 to adjust the amount of light outgoingfrom the lens 1202 may be provided for the lens 1202. An imaging signalprocessing circuit 1205 applies various compensations, clamping andother processing to imaging signals output from the photoelectricconversion apparatus 1204 through a plurality of channels. The imagingsignals output from the imaging signal processing circuit 1205 throughthe plurality of the channels are converted from analog to digital by anA/D converter 1206. A signal processing circuit 1207 (image processingportion) applies various compensations, data compression and otherprocessing to image data output from the A/D converter 1206. Thephotoelectric conversion apparatus 1204, the imaging signal processingcircuit 1205, the A/D converter 1206 and the signal processing circuit1207 operate according to a timing signal generated by a timinggenerator 1208. Each block is controlled by a whole control andarithmetic operation unit 1209. Further, a memory unit 1210 fortemporarily storing the image data and an I/F unit controlling recordingmedium 1211 for recording an image on a recording medium and reading outare provided. A recording medium 1212 includes a semiconductor memoryand is removable. Further, an external I/F unit 1213 for communicatingwith an external computer and the like may be provided. Then, the deviceincluding from the imaging signal processing circuit 1205 to the timinggenerator 1208 may be formed on the same chip as the photoelectricconversion apparatus 1204.

Next, operation shown in FIG. 9 is described. In response to opening ofthe barrier 1201, a main power supply, a power supply for a controlsystem and a power supply for a circuit of an imaging system such as theA/D converter 1206 are turned on in sequence. Subsequently, to controlthe amount of exposing light, the whole control and arithmetic operationunit 1209 opens the diaphragm 1203. A signal output from thephotoelectric conversion apparatus 1204 passes through the imagingsignal processing circuit 1205 and is supplied to the A/D converter1206. The A/D converter 1206 converts the signal from analog to digitaland outputs it to the signal processing circuit 1207. The signalprocessing circuit 1207 processes the data and supplies it to the wholecontrol and arithmetic operation unit 1209, and the whole control andarithmetic operation unit 1209 computes to determine the amount ofexposing light. The whole control and arithmetic operation unit 1209controls the diaphragm based on the amount of exposing light determined.

Next, the whole control and arithmetic operation unit 1209 extracts ahigh-frequency component from the signal output from the photoelectricconversion apparatus 1204 and processed by the signal processing circuit1207, and computes a distance to an object of shooting based on thehigh-frequency component. Subsequently, by driving the lens 1202, it isdetermined whether the lens 1202 is focused or not. When determined tobe not focused, the distance is computed by driving the lens 1202 again.

Then, after confirming focusing, actual exposing is started. After theexposing ends, the imaging signal output from the photoelectricconversion apparatus 1204 is compensated and processed by the imagingsignal processing circuit 1205, converted from analog to digital by theA/D converter 1206 and processed by the signal processing circuit 1207.The image data processed by the signal processing circuit 1207 is storedin the memory unit 1210 by the whole control and arithmetic operationunit 1209. Subsequently, the image data stored in the memory unit 1210is recorded on the recording medium 1212 through the I/F unitcontrolling recording medium by control of the whole control andarithmetic operation unit 1209. Also, the image data is supplied to thecomputer and the like through the external I/F unit 1213 and processed.

The photoelectric conversion apparatus of the present invention isapplied to the imaging system in such a manner. Use of the photoelectricconversion apparatus of the present invention can allow for the drive atthe low voltage, which can reduce power consumption in the imagingsystem. Also, the transfer efficiency of the signal charge is improved,thus being able to provide a better video signal.

The present invention can improve the transfer efficiency by arrangingthe forth semiconductor region 205 of the first conductivity type havinga higher concentration, under most of the first gate electrode 204 andin depth closer to the main surface 212 rather than the secondsemiconductor region 202 forming the photoelectric conversion element101. Also, the voltage value of the first gate electrode 204, instead ofthat of the second semiconductor region 203 of the second conductivitytype on the surface of the photoelectric conversion element 101, iscontrolled, thus suppressing the dark current. Also, the transferefficiency is high and the voltage supplied to the second gate electrodeon transferring the charge, for transferring the charge can be low, andaccordingly, the driving voltage can be set within a voltage range usedin a CMOS process even if a higher concentration is given to the fourthsemiconductor region 205. As the result, the number of saturationsignals can be also increased.

As mentioned above, although the present invention has been describedwith reference to specific embodiments, the present invention should notbe limited to these embodiments. Modifications and combinations may beappropriately made as long as not departing from the spirit and scope ofthe present invention. For example, in the embodiments, the signalcharge has been described as an electron, but it may be an electronhole. In this case, each semiconductor region only has an oppositeconductivity type and the voltage supplied also has an oppositepolarity. Further, the voltage of the low level supplied to the firstgate electrode and the second gate electrode has been the negativevoltage, but this may be a positive voltage.

Also, the low level of the control signal φ204 should not be limited to−1 V, and an electron hole may be accumulated under the first gateelectrode 204. The low level of the control signal φ204 and the controlsignal φ206 can electrically separate the photoelectric conversionelement 101 from the third semiconductor region 207 (in a non-conductingstate), and setting the low level to a voltage value such as −1 V canallow for sufficient separation. Further, the first semiconductor regionand the second semiconductor region may be arranged under the first gateelectrode 204 (the semiconductor region 102).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-123439, filed May 9, 2008, which is hereby incorporated byreference in its entirety.

1. A photoelectric conversion apparatus comprising: a semiconductorsubstrate having a main surface; a first semiconductor region of a firstconductivity type forming a part of a photoelectric conversion element;a second semiconductor region of a second conductivity type opposite tothe first conductivity type being stacked on the first semiconductorregion, and forming a part of the photoelectric conversion element; athird semiconductor region of the first conductivity type to which asignal charge transferred from the photoelectric conversion element; afourth semiconductor region of the first conductivity type having animpurity concentration higher than that of the first semiconductorregion, being arranged between the first and third semiconductor regionand being arranged between the second and third semiconductor regions,and being connected to the first semiconductor region, wherein thebottom of the fourth semiconductor region is closer to the main surfacethan that of the first semiconductor region; a first gate electrodearranged over the fourth semiconductor region, an insulating filmarranged on the main surface and arranged between the first gateelectrode and the fourth semiconductor region; and a second gateelectrode arranged between the third and fourth semiconductor regions,and arranged over the insulating film.
 2. The photoelectric conversionapparatus according to claim 1, wherein each of a plurality of thephotoelectric conversion elements including the first and secondsemiconductor regions is arranged, each of the plurality of thephotoelectric conversion elements is arranged correspondingly to each ofa plurality of the fourth semiconductor regions, and a plurality of thefirst gate electrodes are arranged and connected commonly to a controlline.
 3. The photoelectric conversion apparatus according to claim 1,wherein each of a plurality of the photoelectric conversion elementsincluding the first and second semiconductor regions is arranged, eachof the plurality of the photoelectric conversion elements is arrangedcorrespondingly to each of a plurality of the fourth semiconductorregions, and the first gate electrode is arranged continuously thoughthe plurality of the fourth semiconductor regions.
 4. The photoelectricconversion apparatus according to claim 1, wherein a first pixel and asecond pixel each having the first, second and fourth semiconductorregions and the first and second gate electrodes, such that between thefourth and third semiconductor regions of the first pixel, the fourthsemiconductor region of the second pixel are arranged, and wherein theplurality of the pixels have a common third semiconductor region.
 5. Thephotoelectric conversion apparatus according to claim 4, wherein thesignal charge of the first pixel is transferred to the thirdsemiconductor region through the fourth semiconductor region of thesecond pixel.
 6. An imaging system comprising: a photoelectricconversion apparatus according to claim 1; and a signal processingcircuit for processing a signal output from the photoelectric conversionapparatus.
 7. A driving method of a photoelectric conversion apparatusaccording to claim 1, wherein at least during a period of accumulatingthe signal charge in the photoelectric conversion element, a voltage issupplied to the first gate electrode for accumulating a charge carrierof a polarity opposite to that of the signal charge in the fourthsemiconductor region.
 8. The driving method according to claim 7,wherein the second gate electrode is supplied with a voltage toelectrically disconnect between the third and fourth semiconductorregions, and a value of the voltage to set electrically disconnectbetween the third and fourth semiconductor regions is between a value ofthe voltage supplied to the first gate electrode for accumulating acharge of the polarity opposite to that of the signal charge in thefourth semiconductor region and a value of a voltage to electricallyconnect between the third and fourth semiconductor regions.
 9. Thephotoelectric conversion apparatus according to claim 1, wherein thebottom of the fourth semiconductor region is arranged closer to the mainsurface rather than that of the third semiconductor region.
 10. Thephotoelectric conversion apparatus according to claim 1, wherein thefirst and second gate electrodes are connected to respectively differentdrive wirings.